1. Field of the Invention
The present invention relates to an implantable device which is used to emit electrical stimulation signals to surrounding tissue by means of at least one stimulation electrode, with a sensor unit that generates, from externally fed signals, a useful signal in the form of analogue voltage pulses, and an output stage, which generates stimulation pulses from the useful signal.
2. Related Prior Art
These days, such devices are often used to support or replace certain physiological functions of the human body or to support sensory perception or, for example, to make sensory perception possible again. By way of example, such devices include, but are not limited to, cardiac pacemakers, cochlear implants or retinal implants.
By way of example, active retinal implants, which are to be implanted into the eye, are provided with a multiplicity of stimulation electrodes, which, to retinal cells that are to be contacted, emit electrical stimulation signals. In the process, a multiplicity of pixels convert incident light into the stimulation signals.
By way of example, such a retinal implant is known from WO 2005/000395 A1, the disclosure of which is hereby explicitly incorporated into the present patent application.
The known retinal implant is used to counteract a loss of sight as a result of retinal degeneration. The basic idea is to implant into the eye of a patient a microelectronic stimulation chip, which replaces the lost sight by electrical excitation of nerve cells.
There are two different approaches for possible designs of such retinal prostheses. The subretinal approach uses a stimulation chip, which is implanted into the subretinal space between the outer retina and the pigment epithelium of the retina, and converts ambient light incident on an array of photodiodes integrated in the stimulation chip into stimulation signals for nerve cells. That is to say this retinal implant stimulates the remaining intact neurons of the degenerated retina, i.e., horizontal cells, bipolar cells, amacrine cells and possibly ganglion cells as well.
Thus, the visual image incident on the array of photodiodes or more complex elements is converted into an electrical stimulation pattern, which then, by the “natural computer”, is transmitted to the ganglion cells of the inner retina and from there is lead to the visual cortex via the optic nerve. In other words, the subretinal approach uses the natural interconnections of the once present and now degenerate or lost photoreceptors with the ganglion cells in order to, in the usual way, supply the visual cortex with nerve impulses, which correspond to the visualized image.
By contrast, the epiretinal approach uses a device comprising extra-ocular and intra-ocular parts, which communicate with each other in a suitable fashion. The extra-ocular part comprises a camera and a microelectronic circuit in order to decode received light, that is to say the image information, and to transmit it to the intra-ocular part as a stimulation pattern. The intra-ocular part comprises an electrode array which contacts the neurones of the inner retina and hence directly stimulates the ganglion cells located there.
While the subretinal approach pursues the transmission of light and stimulation of the retina in situ, the image information in the case of the epiretinal approach has to be converted outside into a spatial and temporal stimulation pattern of electrical pulses so that these can be “understood” by the visual cortex.
It is known from a number of publications that transmitting stimulation signals from the stimulation electrodes to the contacted cells requires particular attention. This is due to the fact that the coupling between a stimulation electrode and the contacted tissue is of a capacitive nature; hence, it is only possible to use transient signals for the stimulation. This capacitive coupling is due to the fact that a capacitance (Helmholtz double layer) is formed at the boundary between the electrode and electrolyte in the eye as a result of the electrode polarization.
In the case of the subretinal implant in accordance with WO 2005/000395, mentioned at the outset, the incident light is therefore converted into monophasic anodic voltage pulses with a pulse duration of approximately 500 microseconds and a pulse spacing of preferably 50 ms so that this results in a repetition frequency of 20 Hz, which was found to be sufficient for flicker-free sight and also corresponds to the physiological flicker frequency in the case of low ambient brightness.
Thereby, the pulse spacing of the order of 50 ms is sufficient to be able to reset the electrode polarization. After the stimulation current, which is fed into the tissue by the respective anodic voltage pulse, is emitted, the output of the implant is connected to the electrical ground of the implant by a short-circuit switch so that the capacitance of the Helmholtz double layer discharges again and, averaged over time, there is virtually no charge transport into the tissue.
Humayun, et al., “Pattern Electrical Stimulation of the Human Retina”, Vision Research 39 (1999) 2569-2576, report experiments with epiretinal stimulation, in which so-called biphasic pulses are used which have a cathodic phase, an intermediate phase and an anodic phase of respectively 2 milliseconds. In the case of a stimulation frequency of between 40 and 50 Hz, i.e., significantly above the physiological flicker frequency, flicker-free perception could be observed in two patients.
Jensen, et al., “Responses of Rabbit Retinal Ganglion Cells to Electrical Stimulation with an Epiretinal Electrode”, J. Neural Eng. 2 (2005) 16-21, report the epiretinal excitation of ganglion cells in a rabbit. In the case of anodic and cathodic current pulses lasting 1 millisecond, the authors observed an average latency of the ganglion cells of between 11 and 25 milliseconds for the excitation on the inner retina.
Jensen and Rizzo, “Thresholds for Activation of Rabbit Retinal Ganglion Cells with a Subretinal Electrode”, Experimental Eye Research 2006, 1-7, report subretinal stimulation experiments on an isolated rabbit retina using monophasic current pulses lasting between 0.1 milliseconds and 50 milliseconds, in which they observed latencies of approximately 25 milliseconds.
However, at present, it is not possible to obtain the energy for generating the electrical stimulation signals from the incident useful light itself, even in the case of subretinal implants, so that additional external energy is required. Whereas implants without cables, whose energy is supplied by a co-implanted battery, have been available for quite some time in the case of cardiac pacemakers, many other implants, such as both retinal implants and cochlear implants, require a permanent external supply of energy due to their smaller dimensions and physiological limitations.
In the case of retinal implants, this external energy is either fed by additionally irradiated, non-visible light or it is coupled-in inductively from outside by a coil for example, or it is supplied by means of a cable.
The implant known from WO 2005/000395 A1 is therefore wirelessly supplied with electrical energy by irradiated IR light or by inductively coupled-in RF energy, with it being possible that information for controlling the implant is contained in this externally fed external energy.
US 2004/0181265 A1 discloses a retinal implant which is operated by ambient light only, and which does not use external components for either the energy supply or the provision of image information. The known implant comprises an implanted field of photovoltaic cells, which react to ambient light and generate a supply voltage for a likewise implanted stimulation chip and which are connected to said chip via a cable running within the eye.
The stimulation chip comprises a number of pixels which each have a light sensitive circuit and an electrode connected thereto. The retinal cells are stimulated by means of this electrode using biphasic pulses, which have a pulse duration of 1 ms and a repetition rate of 25 Hz. The biphasic pulses shall be designed in such a way, that, averaged over time, they do not transport charge into the surrounding tissue.
However, since wireless retinal implants for human applications with a sufficient quality are currently not yet available, it is not only epiretinal but also subretinal implants which are currently being used, with the required external energy being supplied to the latter by means of cables.
Gekeler, et al., “Compound subretinal prostheses with extra-ocular parts designed for human trials: successful long-term implantation in pigs”, Graefe's Arch Clin Exp Opthalmol (28 Apr. 2006) [Epub ahead of print], describe, for example, a subretinal retinal implant in which the external energy and the control signals are fed to the chip implanted in the eye via a cable.
Since, on the one hand, DC voltage operated integrated circuits are generally present on the implants, and, on the other hand, since there is little space available on the implants themselves, the known implants are supplied directly with DC voltage. In the case of a supply with AC voltage, the rectifiers needed on the implant would require too much space, particularly due to the necessary smoothing capacitors, and could not be implemented in integrated circuits in a technically sensible fashion either. These problems occur in subretinal retinal implants in particular, but other implants must, of course, also be designed in a space-saving fashion.
However, in the long term, the supply of DC voltage via a cable leads to electrolytic decay processes in the tissue surrounding the cable so that this type of supply of external energy to implants is not satisfactory either.